Nitrogen-based atmospheres have been routinely used by the heat treating industry both in batch and continuous furnaces since mid seventies. Because of low dew point and virtually absence of carbon dioxide, nitrogen-based atmospheres do not exhibit oxidizing and decarburizing properties and are therefore suitable for a variety of heat treating operations. More specifically, a mixture of nitrogen and hydrogen has been extensively used for bright annealing low to high carbon steels and non-ferrous metals and alloys such as copper and gold. A mixture of nitrogen and a hydrocarbon such as methane or propane has gained wide acceptance for neutral hardening and decarburization-free annealing of medium to high carbon steels. A mixture of nitrogen and methanol has been developed and used for carburizing low to medium carbon steels. Finally, a mixture of nitrogen, hydrogen, and moisture has been used for brazing metals and sealing glass to metals.
A major portion of nitrogen used by the heat treating industry has been produced by distillation of air in large cryogenic plants. The cryogenically produced nitrogen is generally pure and expensive. To reduce the cost of nitrogen, several non-cryogenic air separation techniques such as adsorption and permeation have been recently developed and introduced in the market. The non-cryogenically produced nitrogen is much less expensive to produce, but it contains 0.1 to 5% residual oxygen, making a direct substitution of cryogenically produced nitrogen with non-cryogenically produced nitrogen in continuous heat treating furnaces very difficult if not impossible. Non-cryogenically produced nitrogen has, however, been successfully used to replace cryogenically produced nitrogen in applications where surface oxidation can be tolerated. For example, non-cryogenically produced nitrogen has been successfully used in oxide annealing of carbon steel parts which are generally machined after heat treatment.
Non-cryogenically produced nitrogen has also been successfully used to replace cryogenically produced nitrogen in applications where surface finish or appearance is not important. For example, a mixture of non-cryogenically produced nitrogen and methanol is being used commercially for carburizing steels in batch furnaces as well as continuous furnaces equipped with integrated quench cooling zones. Carburizing processes based on non-cryogenically produced nitrogen/methanol mixture have been described in detail in papers titled, "Carburizing with Membrane N.sub.2 : Process and Quality Issues", published in Heat Treating, pages 28-32, March 1988, "New Method of Generating Nitrogen for Controlled Atmosphere Heat Treatment at Torrington Shiloh Plant", published in Industrial Heating, pages 40-46, March 1986, and "The Use of Non-Cryogenically Produced Nitrogen in Furnace Atmospheres", published in Heat Treatment of Metals, pages 63-67, March 1989. The use of non-cryogenically produced nitrogen/methanol mixture has, however, not been successful for bright annealing or heat treating parts without surface oxidation in continuous furnaces or for heat treating parts in continuous furnaces operated at temperatures below about 900.degree. C. These systems are not cost-effective because they require expensive methanol delivery system.
Attempts, with limited success, have been made to use other reducing gases such as a hydrocarbon and hydrogen along with non-cryogenically produced nitrogen to produce atmospheres suitable for heat treating or bright annealing parts in continuous furnaces. The problem has generally been related to surface oxidation of the heat treated or annealed parts in the cooling and/or heating zones of the furnace. For example, methane has been added into the heating zone of continuous furnaces with non-cryogenically produced nitrogen in an attempt to generate atmospheres suitable for oxidation- and decarburization-free annealing or hardening of carbon steels. This method was not successful due to excessive oxidation and decarburization of the parts, as described in a paper titled, "The Use of Non-Cryogenically Produced Nitrogen in Furnace Atmospheres", published in Heat Treatment of Metals, pages 63-67, March 1989. The oxidation and decarburization problems were related to the slow rate of reaction between oxygen and methane at low temperatures and/or short residence times in continuous furnaces used for oxide- and decarburize-free annealing. The above paper concluded that non-cryogenically produced nitrogen would be cost competitive to cryogenically produced nitrogen only at residual oxygen levels below about 0.2%, if at all possible. Attempts have also been made to use hydrogen gas as a reducing agent with non-cryogenically produced nitrogen for oxide-free and bright annealing of carbon steels in continuous furnaces. Unfortunately, the process required excessive amounts of hydrogen, making the use of non-cryogenically produced nitrogen economically unattractive.
A mixture of non-cryogenically produced nitrogen and hydrogen has been used for annealing copper and described in papers titled, "The Use of Non-Cryogenically Produced Nitrogen in Furnace Atmospheres", published in Heat Treatment of Metals, pages 63-67, March 1989 and "A Cost Effective Nitrogen-Based Atmosphere for Copper Annealing", published in Heat Treatment of Metals, pages 93-97, April 1990. These papers describe that heat treated copper product was slightly discolored when all the gaseous feed containing a mixture of hydrogen and non-cryogenically produced nitrogen with residual oxygen was introduced into the hot zone of a continuous furnace. It is, therefore, clearly evident that according to the prior art, copper cannot be bright annealed with a mixture of non-cryogenically produced nitrogen and hydrogen in continuous furnaces.
U.S. Pat. No. 5,057,164 discloses and claims a process producing atmospheres suitable for heat treating metals from non-cryogenically produced nitrogen in continuous furnaces by reacting residual oxygen with hydrogen or carbon monoxide in the heating zone followed by removing a part of the atmosphere from the heating zone and introducing it into the cooling zone of the furnace. Unfortunately, this process requires large amounts of hydrogen or carbon monoxide to provide high pH.sub.2 /pH.sub.2 O or pCO/pCO.sub.2 ratio (or reducing environment) in the furnace, making it uneconomical for bright annealing steels, brazing steels, and sintering steel powders in continuous furnaces.
Researchers have explored numerous alternative ways of using non-cryogenically produced nitrogen for heat treating metals in continuous furnaces. For example, furnace atmospheres suitable for bright annealing copper, brazing copper, decarburized annealing steel, and sintering copper and copper alloys have been claimed to be generated from non-cryogenically produced nitrogen by converting residual oxygen to moisture with hydrogen gas in external units prior to feeding atmospheres into the furnaces. Such atmosphere generation methods have been disclosed in detail in U.S. Pat. No. 3,535,074, Australian Patent Applications AU45561/89 and AU45562/89 dated 24 Nov. 1988, and European Patent Application 90306645.4 dated 19 Jun. 1990. Unfortunately, these processes are not cost-effective for bright annealing steels, brazing steels, and sintering steel powders because they require large amounts of hydrogen to maintain high pH.sub.2 /pH.sub.2 O ratio (or reducing environment) in the furnace. Additionally, these processes are limited in scope because they result in excessive decarburization of steel parts.
U.S. Pat. No. 4,931,070 and French patent publications 2,639,249 and 2,639,251 dated 24 Nov. 1988 claim producing atmospheres suitable for heat treating metals from non-cryogenically produced nitrogen by converting residual oxygen to moisture with hydrogen in external catalytic units followed by extraction of moisture prior to introducing atmospheres into furnaces. These methods are not cost effective because of the substantial costs involved in removing moisture from atmospheres.
U.S. Pat. No. 5,069,728 discloses and claims a method of producing atmospheres suitable for heat treating from non-cryogenically produced nitrogen by simultaneously introducing non-cryogenically produced nitrogen along with hydrogen and carbon monoxide into the heating zone and non-cryogenically produced nitrogen pre-treated to convert the residual oxygen to moisture with hydrogen in an external catalytic reactor or nitrogen gas free of oxygen into the cooling zone of a continuous furnace. Unfortunately, this method requires large amounts of hydrogen or carbon monoxide to maintain high pH.sub.2 /pH.sub.2 O or pCO/pCO.sub.2 ratio (or reducing environment) in the furnace, making it uneconomical for bright annealing steels, brazing steels, and sintering steel powders in continuous furnaces.
U.S. Pat. No. 4,859,434 claims producing atmospheres suitable for heat treating from non-cryogenically produced nitrogen by reacting residual oxygen with vaporized methanol in an external catalytic reactor. This method has not been used in many parts of the world because of the costs involved in installing a methanol delivery system.
Based upon the above discussion, it is clear that there is a need to develop a process for generating low-cost atmospheres for bright annealing ferrous and non-ferrous metals and alloys, brazing steels, sealing glass to metals, neutral hardening low, medium, and high carbon steels, and sintering metal and ceramic powders in continuous furnaces from non-cryogenically produced nitrogen. Additionally, there is a need to develop a process which 1) is cost effective, 2) eliminates need of both expensive hydrogen and delivery system for the reducing agent, and 3) minimizes or prevents decarburization of steels during heat treatment.